A bar code is a coded pattern of graphical indicia having a series or pattern of bars and spaces of varying widths that encode information. Bar codes may be one dimensional (e.g., UPC bar code) or two dimensional (e.g., DataMatrix bar code). Systems that read, that is, image and decode bar codes employing imaging camera systems are typically referred to as imaging-based bar code readers or bar code scanners.
Imaging-based bar code readers may be portable or stationary. Stationary bar code readers are mounted in a fixed position and a target object, e.g., a product package that includes a target bar code, is moved or swiped past a transparent window so the object passes through a field of view of the stationary bar code reader. The bar code reader typically provides an audible and/or visual signal to indicate the target bar code has been successfully imaged and decoded.
A typical example where a stationary imaging-based bar code reader would be utilized includes a point of sale counter/cash register where customers pay for their purchases. The bar code reader is typically enclosed in a housing that is installed in a counter and normally includes a vertically oriented transparent window and/or a horizontally oriented transparent window, either of which may be used for reading the target bar code affixed to the target object, i.e., the product or product packaging. The sales person (or customer in the case of self-service check out) sequentially presents each target object's bar code either to the vertically oriented window or the horizontally oriented window, whichever is more convenient given the specific size and shape of the target object and the position of the bar code on the target object.
A stationary imaging-based bar code reader that has a plurality of imaging cameras can be referred to as a multi-camera imaging-based scanner or bar code reader. In a multi-camera imaging reader, each camera system typically is positioned so that it has a different field of view from every other camera system. While the fields of view may overlap to some degree, the effective or total field of view of the reader is increased by adding additional camera systems. Hence, the desirability of multi-camera readers as compared to single camera bar code readers which have a smaller effective field of view and require presentation of a target bar code to the reader in a very limited orientation to obtain a successful, decodable image.
If several imaging cameras are positioned to look out of the bar code reader's window or windows in different directions, the bar code reader can decode barcodes on various sides of packages that are swiped through the multiple, overlapping fields of view. Swiping promotes higher throughput and also assures that all the cameras have the opportunity to see their respective sides of a package, which may not happen if the package is stationary. Existing laser-based supermarket scanners are designed to handle swipe speeds up to 100 inches per second. With this high swipe speed, the barcode may only be within the field of view of any given camera for a single camera exposure frame, so it becomes important that the single frame be properly exposed, or the barcode might not be decodable.
One way to accommodate signal variations due to barcode distance and window composition is to use an auto-exposure system. Many camera image sensors include internal auto-exposure circuitry. This circuitry measures the level of light that is received on one frame and adjusts exposure duration on the next subsequent frame based on measurements taken on the first image or first frame. The result is that the exposure of the first frame may not be adequate to decode a barcode image but that the second captured frame should produce a decodable image.
Such adjustments are acceptable for some image readers, such as handheld bar code readers where the barcode remains stationary in front of the reader for several frame durations. Such an adjustment is not acceptable, however, where the user swipes the barcode rapidly through a field of view of the camera (or cameras) of a barcode reader. When swipe speed is high, the barcode can pass through the field of view before a second frame can be captured so it is important that the first frame captured be of the proper exposure for bar code interpretation.
U.S. Pat. No. 7,357,325 discusses an ‘Automatic Light Exposure Measurement and Illumination Control Subsystem’ that utilizes a rolling shutter with a single camera. It measures, in real-time, a power density [joules/cm] of photonic energy (i.e. light) collected by the optics of the system at its image sensing array and generates Auto-Exposure Control Signals indicating the amount of exposure required for good image formation and detection. The system of this patent uses an optical band pass filter in the imaging system. An optical filter is needed with a rolling shutter since parts of the sensor will be exposing for long periods of time when the illumination is turned off, so these parts would provide blurred images if ambient light illuminated a moving barcode during that period.
Global shutters for use with imaging barcode readers and scan engines are known. The duration of the illumination matches or exceeds the integration or exposure time. Some global shutter scanners activate the illumination only during the integration time while others have the illumination activated continuously.